Infusoria
in-fu-so'ri-a, minute animals abounding in waters that contain decaying animal and vegetable matter. They are protozoa which are ciliated either through life or in the young condition. Some authorities therefore call them ciliata. The form of the body varies. Like other protozoa it consists of a single cell, which may be roundish, kidney-shaped, ovoid, vase-shaped, trumpet-shaped, or flattened. Some are free-swimming; others are attached to stones, weeds, or sticks by a stalk. Still others are parasitic. In some species the stalk is quite firm; in others it is very contractile, and can coil like a cork-screw when the animal is stimulated in any way.
Like the amoeba, the body consists of soft, transparent protoplasm; but, unlike the amoeba, this protoplasm is surrounded by a thin but firm wall, so that the shape of the body can not be so readily altered. From the outer layer of the protoplasm fine, thread-like projections extend through the wall into the water in which the animal lives. These thread-like projections are called cilia (singular, cilium). The cilia aid in locomotion and also in getting food. The arrangement of the cilia is not the same in all infusoria. In some species the cilia are small, nearly equal in size, and distributed in rows all over the body of the animal. In others the cilia are of unequal size, the larger being found usually near the place where food is taken in; the rest of the body being covered with small cilia. In still others a row of large cilia is found near the mouth-end, the rest of the body being quite naked. In the so-called highest form of infusoria the cilia take the form of hooks, fans, bristles, and plates with fringed ends, and are much bigger and stronger than ordinary cilia. They are, therefore, used by the animal much as an insect uses its legs; and it is a highly interesting and amusing sight to see, under a powerful magnifying lens, a few of these tiny creatures apparently crawling about on bits of algae.
In addition to cilia some forms possess various other external structures. In some forms minute sacs lie side by side in the cortex or outer layer of the protoplasm. A long, whiplash-like thread or cilium is coiled in each of these sacs, and when the animal is irritated in some way these whiplashes are quickly thrown out in defense.
Infusoria contain one or more nuclei of various shapes. The number of contractile vacuoles also vary from one to a hundred or more. These may be scattered all over the cortex or arranged in one or two rows. In some forms the vacuoles have a star-like arrangement, and it is a most fascinating occupation to watch these vacuoles fill, and then disappear, ten times or more in one minute. It is thought that the animal gets rid of surplus water or of excretions in the body by means of these vacuoles.
Infusoria do not ingest food by surrounding it, as do the amoebae. In forms which are not parasitic there is a definite mouth or depression through which food enters. Food consists of small algae or other small organic particles found in water. In some forms there is also a definite place through which waste matter is ejected; but a definite anus seems to be lacking.
Stalked infusoria can move only as far as the length of their stalk permits; but free infusoria move rapidly through the water, either with a straight, shooting motion, or in a wavy, undulating fashion. There seems to be as great a difference in manner of movement among infusoria as there is difference in manner of movement among birds. Both make an interesting study.
Reproduction among infusoria takes place almost exclusively by means of a simple division or fission, one animal dividing into two equal or nearly equal parts.
The infusoria received their name from the fact that they are usually found in infusions; that is, in water in which hay, grass, or other organic material is in soak. Some interesting forms can be gotten for observation in one's own room by placing a handful of hay in a jar and pouring cold water over it. In a few days the water will be teeming with miscroscopic ciliated animals. Infusoria are found in rivers, lakes, ponds, or streams, and also in the salt water of ocean or sea.--ELLEN TORELLE.